In recent years wavelength division multiplexed (WDM) optical transmission systems have been increasingly deployed in optical networks. These include coarse wavelength division multiplexed (CWDM) and dense wavelength division multiplexed (DWDM) systems. Whether a system is considered to be CWDM or DWDM simply depends upon the optical frequency spacing of the channels utilized in the system. Although WDM optical transmission systems have increased the speed and capacity of optical networks, the performance of such systems is limited by various factors such as chromatic dispersion and the fiber nonlinearity, which can cause pulse shape change in the case of baseband digital signals and distortions in case of analog signals. These impairments degrade the quality of the optically transmitted information. Fiber nonlinearities, for example, can give rise to crosstalk between optical signals operating at different wavelengths. When crosstalk occurs, modulation components of one signal are superimposed on another signal at a different wavelength. If the level of crosstalk is sufficiently large it will corrupt the information being transmitted by the optical signals impacted by this impairment.
One common cause of crosstalk, in an optical fiber communication system with multiple wavelengths, is Raman scattering. This type of crosstalk is caused by stimulated Raman scattering (SRS) in silica fibers (and other materials) when a pump wave co-propagates with a signal wave through it. Stimulated Raman scattering is an inelastic scattering process in which an incident pump photon loses its energy to create another photon of reduced energy at a lower frequency. The remaining energy is absorbed by the fiber medium in the form of molecular vibrations (i.e. optical phonons) FIG. (1) is a schematic diagram of the stimulated Raman scattering process. The picture depicts a pump photon scattering in the Raman media. As a result of the scattering event the pump photon is annihilated and a new signal photon at the Stokes frequency is created along with an optical phonon at the Stokes shift frequency. Both energy and momentum are conserved:ωpump=ωsignal+ωOp phonon and {right arrow over (k)}pump={right arrow over (k)}signal+{right arrow over (k)}Op phonon,where ωx is the frequency of x and kx is the associated wavevector of x and  is Planck's constant divided by 2π.
The difference between the optical frequency of the pumping wave (the higher frequency) and the wave being amplified is referred to as the Stokes shift. The Raman gain window of a typical silica fiber is about 25 THz (terahertz) wide. The Stokes shift for the maximum Raman power transfer, in a typical silica fiber, is approximately 13 THz. The Raman gain between two optical signals increases from zero as the frequency separation between the two signal increases until the peak gain is reached at a 13 THz separation. It then decreases back to zero as the separation increases beyond 25 THz. As a result of SRS, energy from the pump wavelength can amplify a signal at a longer wavelength (lower optical frequency) so long as the optical frequency separation between the two signals falls within the Raman gain window of the fiber. The pumping wave loses energy (pump photon annihilation) to the signal wavelength (Stokes shifted photon creation) and also to the fiber (optical phonon creation). Thus, due to SRS, the pump amplitude decreases as its photon population depletes while the signal wavelength is amplified as its photon population increases.
SRS amplification is a benign process if the pump wave is not modulated with any type of information that would cause its amplitude to change with time. In this case the Raman amplification is fixed at a constant level over time. This simply serves to boost the amplitude of the Stokes shifted signal but does not disturb the information that it might be carrying. Problems arise, however, when both the pump and signal wavelengths are modulated. In this case the amplitude of the pumping wavelength is changing over time and, hence, the SRS amplification level varies in concert with the pump modulation. This process imparts a scaled replica of the pump modulation onto the signal wavelength which is referred to as crosstalk. The crosstalk can interfere with and degrade the quality of the original information being transmitted by the signal wavelength. The level of the crosstalk modulation depends upon the Raman gain value, which, in turn, depends upon the optical frequency separation of the transmitted waves, amongst other parameters. Furthermore, the crosstalk process is a shared experience between the pump and signal wavelengths when they are both modulated. It is more probable that a pump photon will scatter and be annihilated in an SRS event when there are more signal photons available to facilitate the process. Therefore, at points in time-space where the signal wavelength is peaked, due to the modulation it carries, the pump will more easily lose photons during SRS. The reverse is true, as well, at points in time-space where the signal wavelength is smaller due to the time varying modulation it carries the pump is less likely to lose a photon in an SRS event. Hence the pump loses photons in concert with the signal wavelength's modulation. The result is that an inverted scaled replica of the signal wavelength's modulation is imparted upon the pump wavelength. This crosstalk can interfere with and degrade the quality of the original information being transmitted by the pump wavelength.
FIG. 2 shows how this transfer of energy gives rise to crosstalk. FIG. 2 is a simplified illustration that is useful in facilitating an understanding of Raman crosstalk between two optical channels or signals Si and Sj, where Sj is at a longer wavelength than Si. FIG. 2A shows the signal Si and FIG. 2B shows the signal Sj. For simplicity of illustration Sj is shown as a signal with constant amplitude (i.e. a continuous string of zeros or ones). As indicated in FIG. 2C, the pattern of signal Si (dashed line) is impressed on the signal Sj by the process of Raman amplification. In other words, signal Sj now includes as one of its components the pattern of signal Si. Likewise, since signal Si is pumping the signal Sj, the pattern of signal Sj (had it been modulated) would be impressed upon the pump Si by the process of pump depletion. In a multiple wavelength system (three or more wavelengths) the crosstalk process is similar to what has already been described but the complexity grows since now there are multiple pumping sources and multiple signals generating crosstalk on each of the optical waves. The Raman crosstalk is a problem for both analog and digital modulation schemes.
In addition to the generation of unwanted crosstalk the SRS process can also lead to the generation of Raman induced second order (CSO: composite second order) and third order (CTB: composite triple beat) distortions. These distortions occur as result of the nonlinear nature of the Raman amplification process which, in the undepleted regime, is exponential in form. Suppose there are two optical waves at wavelengths λs (the signal wavelength) and λp (the pump wavelength) propagating through a fiber of length L with a corresponding Raman gain coefficient Gsp. If at the transmitter site the instantaneous optical power associated with λp is Pp(t) and the instantaneous optical power associated with λs is Ps(t) then, in the undepleted power regime, the optical power at the wavelength λs at position L due to the Raman scattering is given by:Ps(t,L)=Ps(t)e[Gsp ρLLeffPp(t)−αL]if λp<λs  (1a)or
                                          P            s                    ⁡                      (                          t              ,              L                        )                          =                                                            P                s                            ⁡                              (                t                )                                      ⁢                          ⅇ                              [                                                                            -                                              G                        sp                                                              ⁢                                          ρ                      _                                        ⁢                                                                                  ⁢                                          L                                                                                                                        eff                                                              ⁢                                                                                                                        n                            ~                                                    p                                                ⁢                                                  λ                          p                                                                                                                                                  n                            ~                                                    s                                                ⁢                                                  λ                          s                                                                                      ⁢                                                                  P                        p                                            ⁡                                              (                        t                        )                                                                              -                                      α                    ⁢                                                                                  ⁢                    L                                                  ]                                      ⁢                                                  ⁢            if            ⁢                                                  ⁢                          λ              s                                <                      λ            p                                              (                  1          ⁢          b                )            
Here Leff is the effective length in the fiber at the pump wavelength, α is the power attenuation factor in the fiber at the signal wavelength, ρL is the running average probability of finding the two signals in the same state of polarization, ñp and ñs are the indices of refraction at the respective wavelengths. By defining a simple function Hs,p:
                              H                      s            ,            p                          ≡                  {                                                    1                                                                                  λ                    s                                    >                                      λ                    p                                                                                                                                            -                    1                                    ⁢                                                                                                              n                          ~                                                p                                            ⁢                                              λ                        p                                                                                                                                      n                          ~                                                s                                            ⁢                                              λ                        s                                                                                                                                                              λ                    s                                    <                                      λ                    p                                                                                                          (                  1          ⁢          c                ⁢                                  )            Then (1a) and (1b) can be combined into a single equation:Ps(t,L)=Ps(t)e[HspGsp ρLLeffPp(t)−αL]  (1d)This will come in handy when addressing a multi-wavelength optical communication system.
Expanding the exponential in (1a) gives:
                                                                                                                 P                    s                                    ⁡                                      (                                          t                      ,                      L                                        )                                                  =                                ⁢                                                                                                    P                        s                                            ⁡                                              (                        t                        )                                                              ⁡                                          [                                              1                        +                                                                              G                            sp                                                    ⁢                                                                                    ρ                              _                                                        L                                                    ⁢                                                      L                            eff                                                    ⁢                                                                                    P                              p                                                        ⁡                                                          (                              t                              )                                                                                                      +                                                                                                                                            (                                                                                                      G                                    sp                                                                    ⁢                                                                                                            ρ                                      _                                                                        L                                                                    ⁢                                                                      L                                    eff                                                                                                  )                                                            2                                                        2                                                    ⁢                                                                                    (                                                                                                P                                  p                                                                ⁡                                                                  (                                  t                                  )                                                                                            )                                                        2                                                                          +                        …                                            ]                                                        ⁢                                      ⅇ                                                                  -                        α                                            ⁢                                                                                          ⁢                      L                                                                                                                                              =                                ⁢                                                      [                                                                                            P                          s                                                ⁡                                                  (                          t                          )                                                                    +                                                                        G                          sp                                                ⁢                                                                              ρ                            _                                                    L                                                ⁢                                                  L                          eff                                                ⁢                                                                              P                            s                                                    ⁡                                                      (                            t                            )                                                                          ⁢                                                                              P                            p                                                    ⁡                                                      (                            t                            )                                                                                              +                                                                                                                                  (                                                                                                G                                  sp                                                                ⁢                                                                                                      ρ                                    _                                                                    L                                                                ⁢                                                                  L                                  eff                                                                                            )                                                        2                                                    2                                                ⁢                                                                              P                            s                                                    ⁡                                                      (                            t                            )                                                                          ⁢                                                                              (                                                                                          P                                p                                                            ⁡                                                              (                                t                                )                                                                                      )                                                    2                                                                    +                      …                                        ]                                    ⁢                                      ⅇ                                                                  -                        α                                            ⁢                                                                                          ⁢                      L                                                                                                                                (          2          )                    
The second line of (2) provides the sought after explanation to the Raman induced crosstalk, CSO, and CTB distortions in the near zero dispersion optical communication system when the time dependent pump and signal wave powers are represented by:
                                                                                          P                  s                                ⁡                                  (                  t                  )                                            =                                                P                                      0                    ⁢                    s                                                  +                                                      P                    ms                                    ⁡                                      (                    t                    )                                                                                                                          =                                                P                                      0                    ⁢                    s                                                  +                                                      P                                          0                      ⁢                      s                                                        ⁢                                      m                    s                                    ⁢                                                            f                      s                                        ⁡                                          (                      t                      )                                                                      +                                  CSO                  s                                +                                  CTB                  s                                                                                        (                  3          ⁢          a                )            
                                                                                          P                  p                                ⁡                                  (                  t                  )                                            =                                                P                                      0                    ⁢                    p                                                  +                                                      P                    mp                                    ⁡                                      (                    t                    )                                                                                                                          =                                                P                                      0                    ⁢                    p                                                  +                                                      P                                          0                      ⁢                      p                                                        ⁢                                      m                    p                                    ⁢                                                            f                      p                                        ⁡                                          (                      t                      )                                                                      +                                  CSO                  p                                +                                  CTB                  p                                                                                        (                  3          ⁢          b                )            
Here P0s, P0p are the average optical powers of the signal and pump waves, Pms(t), Pmp(t) represent the explicitly time dependent terms of the optical powers, and ms, mp are the respective optical modulation indices (OMI) for each laser. The third and fourth terms of the second lines in (3a) and (3b) represent the composite second order (CSOs, CSOp) and composite triple beat (CTBs, CTBp) distortions generated within the signal and pump optical transmitters themselves. The CSOs and CSOp distortions are native to the transmitters and are independent of the Raman interactions taking place in the fiber. The time dependent modulating functions fs(t) and fp(t) represented the information being carried on each optical wave. After substituting (3a) and (3b) into (2) and retaining only the most dominant terms one obtains:
                                          P            s                    ⁡                      (                          t              ,              L                        )                          =                                                            P                                  0                  ⁢                  s                                            ⁡                              [                                  1                  +                                                            G                      sp                                        ⁢                                                                  ρ                        _                                            L                                        ⁢                                          L                      eff                                        ⁢                                          P                                              0                        ⁢                        p                                                                                            ]                                      ⁢                          ⅇ                                                -                  α                                ⁢                                                                  ⁢                L                                              +                                    P                              0                ⁢                s                                      ⁢                          m              s                        ⁢                                                            f                  s                                ⁡                                  (                  t                  )                                            ⁡                              [                                  1                  +                                                            G                      sp                                        ⁢                                                                  ρ                        _                                            L                                        ⁢                                          L                      eff                                        ⁢                                          P                                              0                        ⁢                        p                                                                                            ]                                      ⁢                          ⅇ                                                -                  α                                ⁢                                                                  ⁢                L                                              +                                    G              sp                        ⁢                                          ρ                _                            L                        ⁢                          L              eff                        ⁢                                          P                                  0                  ⁢                  s                                            ⁡                              (                                                      P                                          0                      ⁢                      p                                                        ⁢                                      m                    p                                    ⁢                                                            f                      p                                        ⁡                                          (                      t                      )                                                                      )                                      ⁢                          ⅇ                                                -                  α                                ⁢                                                                  ⁢                L                                              +                                                    CSO                s                            ⁡                              [                                  1                  +                                                            G                      sp                                        ⁢                                                                  ρ                        _                                            L                                        ⁢                                          L                      eff                                        ⁢                                          P                                              0                        ⁢                        p                                                                                            ]                                      ⁢                          ⅇ                                                -                  α                                ⁢                                                                  ⁢                L                                              +                                    G              sp                        ⁢                                          ρ                _                            L                        ⁢                          L              eff                        ⁢                          P                              0                ⁢                s                                      ⁢                          CSO              p                        ⁢                          ⅇ                                                -                  α                                ⁢                                                                  ⁢                L                                              +                                    G              sp                        ⁢                                          ρ                _                            L                        ⁢                                                            L                  eff                                ⁡                                  [                                                            P                                              0                        ⁢                        s                                                              ⁢                                          m                      s                                        ⁢                                                                  f                                                  s                          ⁢                                                                                                                                                    ⁡                                              (                        t                        )                                                                              ]                                            ⁡                              [                                                      P                                          0                      ⁢                      p                                                        ⁢                                      m                    p                                    ⁢                                                            f                      p                                        ⁡                                          (                      t                      )                                                                      ]                                      ⁢                          ⅇ                                                -                  α                                ⁢                                                                  ⁢                L                                              +                                                    CTB                s                            ⁡                              [                                  1                  +                                                            G                      sp                                        ⁢                                                                  ρ                        _                                            L                                        ⁢                                          L                      eff                                        ⁢                                          P                                              0                        ⁢                        p                                                                                            ]                                      ⁢                          ⅇ                                                -                  α                                ⁢                                                                  ⁢                L                                              +                                    G              sp                        ⁢                                          ρ                _                            L                        ⁢                          L              eff                        ⁢                          P                              0                ⁢                s                                      ⁢                          CTB              p                        ⁢                          ⅇ                                                -                  α                                ⁢                                                                  ⁢                L                                              +                                                                                                                                        (                                                                              G                            sp                                                    ⁢                                                                                    ρ                              _                                                        L                                                    ⁢                                                      L                            eff                                                                          )                                            2                                        2                                    ⁡                                      [                                                                  P                                                  0                          ⁢                          s                                                                    ⁢                                              m                        s                                            ⁢                                                                        f                          s                                                ⁡                                                  (                          t                          )                                                                                      ]                                                  ⁡                                  [                                                            P                                              0                        ⁢                        p                                                              ⁢                                          m                      p                                        ⁢                                                                  f                        p                                            ⁡                                              (                        t                        )                                                                              ]                                            2                        ⁢                          ⅇ                                                -                  α                                ⁢                                                                  ⁢                L                                                                        (        4        )            
The second line of (4) contains the undistorted signal transmitter's RF subcarrier modulation multiplied by the Raman gain term (1+Gsp ρLLeffP0p) and an additional first order RF subcarrier term arising from the modulated pump laser. This additional first order term is RF subcarrier crosstalk, a direct transfer of the pump laser's RF subcarrier modulation (P0pmpfp(t)) to the signal carrier scaled by the Raman factor Gsp ρLLeffP0s. When the RF subcarrier crosstalk is exactly in phase the signals constructively add (plus sign) while if they are exactly out of phase they destructively interfere (minus sign), all other phasing possibilities fall between these two extremes. The first terms of the third and fourth lines are the signal laser's generated CSOs and CTBs terms each multiplied by the same Raman gain term as the signal laser's RF subcarrier modulation. The second terms of the third and fourth lines are respectively the direct transfer of the pump laser's CSOp and CTBp distortions (distortion crosstalk) to the signal wave scaled by the same Raman factor as the RF subcarrier crosstalk that is transferred from the pump wave to the signal wave. The third terms of the third and fourth lines are the new Raman generated CSOR and CTBR distortions resulting from the product of the signal and pump lasers' RF subcarrier modulations. Collectively, the RF subcarrier crosstalk term, along with the second and third terms of the third and fourth lines constitute degraded performance of the signal transmitter due to the Raman interactions between the modulated pump and signal lasers. These will be denoted collectively as the Raman induced distortions. If the pump and signal lasers' modulations are not in phase with one another interference can also result between the various terms within each line of (4). It is the purpose of this invention to elucidate the methods and apparatus that can be utilized to reduce the deleterious effects of the Raman induced RF subcarrier crosstalk and CSO distortion terms. The Raman induced CTB is insignificant in magnitude and not treated here. When there are three or more lasers in the system (4) must be modified to account for the multiple pumps. For a system with n transmitters (4) becomes:
                                          P            s                    ⁡                      (                          t              ,              L                        )                          =                                                            P                                  0                  ⁢                  s                                            (                              1                +                                                      L                    eff                                    ⁢                                                            ∑                                              p                        =                        1                                            n                                        ⁢                                                                  H                                                  s                          ,                          p                                                                    ⁢                                              G                                                  s                          ,                          p                                                                    ⁢                                                                        ρ                          _                                                                          Ls                          ,                          p                                                                    ⁢                                              P                                                  0                          ⁢                          p                                                                                                                                )                        ⁢                                                  ⁢                          ⅇ                                                -                  α                                ⁢                                                                  ⁢                L                                              +                                          ⁢                                          ⁢                                    [                                                                    P                                          0                      ⁢                      s                                                        ⁢                                                                          ⁢                                      m                    s                                    ⁢                                                            f                      s                                        ⁡                                          (                      t                      )                                                        ⁢                                      (                                          1                      +                                                                        L                          eff                                                ⁢                                                                              ∑                                                          p                              =                              1                                                        n                                                    ⁢                                                                                    H                                                              s                                ,                                p                                                                                      ⁢                                                          G                                                              s                                ,                                p                                                                                      ⁢                                                                                          ρ                                _                                                                                            Ls                                ,                                p                                                                                      ⁢                                                          P                                                              0                                ⁢                                p                                                                                                                                                                          )                                                  +                                                      P                                          0                      ⁢                      s                                                        ⁢                                      L                    eff                                    ⁢                                                            ∑                                              p                        =                        1                                            n                                        ⁢                                                                  H                                                  s                          ,                          p                                                                    ⁢                                              P                                                  0                          ⁢                          p                                                                    ⁢                                              m                        p                                            ⁢                                                                        f                          p                                                ⁡                                                  (                          t                          )                                                                    ⁢                                              G                                                  s                          ,                          p                                                                    ⁢                                                                        ρ                          _                                                                          Ls                          ,                          p                                                                                                                                ]                        ⁢                                                  ⁢                                                            ⅇ                  ⁢                                                                                                                              -                    α                                    ⁢                                                                          ⁢                  L                                            ⁢                                                          [                                                                    CSO                    s                                    (                                      1                    +                                                                  L                        eff                                            ⁢                                                                        ∑                                                      p                            =                            1                                                    n                                                ⁢                                                                              H                                                          s                              ,                              p                                                                                ⁢                                                      G                            sp                                                    ⁢                                                                                    ρ                              _                                                        L                                                    ⁢                                                      P                                                          0                              ⁢                              p                                                                                                                                                            )                                +                                                      P                                          0                      ⁢                      s                                                        ⁢                                      L                    eff                                    ⁢                                                            ∑                                              p                        =                        1                                            n                                        ⁢                                                                  H                                                                              s                            ,                            p                                                    ⁢                                                                                                                                                    ⁢                                              G                                                  s                          ,                          p                                                                    ⁢                                                                        ρ                          _                                                                          Ls                          ,                          p                                                                    ⁢                                              CSO                        p                                                                                            +                                                      P                                          0                      ⁢                      s                                                        ⁢                                      m                    s                                    ⁢                                                            f                      s                                        ⁡                                          (                      t                      )                                                        ⁢                                      L                    eff                                    ⁢                                                            ∑                                              p                        =                        1                                            n                                        ⁢                                                                  H                                                  s                          ,                          p                                                                    ⁢                                              G                                                  s                          ,                          p                                                                    ⁢                                                                        ρ                          _                                                                          Ls                          ,                          p                                                                    ⁢                                              P                                                  0                          ⁢                          p                                                                    ⁢                                              m                        p                                            ⁢                                                                        f                          p                                                ⁡                                                  (                          t                          )                                                                                                                                ]                        ⁢                                          ⅇ                                                      -                    α                                    ⁢                                                                          ⁢                  L                                            ⁢                                                          [                                                                    CTB                    s                                    (                                      1                    +                                                                  L                        eff                                            ⁢                                                                        ∑                                                      p                            =                            1                                                    n                                                ⁢                                                                              H                                                          s                              ,                              p                                                                                ⁢                                                      G                                                          s                              ,                              p                                                                                ⁢                                                                                    ρ                              _                                                                                      Ls                              ,                              p                                                                                ⁢                                                      P                                                          0                              ⁢                              p                                                                                                                                                            )                                +                                                      P                                          0                      ⁢                      s                                                        ⁢                                      L                    eff                                    ⁢                                                            ∑                                              p                        =                        1                                            n                                        ⁢                                                                  H                                                  s                          ,                          p                                                                    ⁢                                              G                                                  s                          ,                          p                                                                    ⁢                                                                        ρ                          _                                                                                                                                                          ⁢                                                      Ls                            ,                            p                                                                                              ⁢                                              CTB                        p                                                                                            +                                                      P                                          0                      ⁢                      s                                                        ⁢                                      m                    s                                    ⁢                                                            f                      s                                        ⁡                                          (                      t                      )                                                        ⁢                                                                                                                                          (                                                          L                              eff                                                        )                                                    2                                                2                                            [                                                                        ∑                                                      p                            =                            1                                                    n                                                ⁢                                                                              H                                                          s                              ,                              p                                                                                ⁢                                                      G                                                          s                              ,                              p                                                                                ⁢                                                                                    ρ                              _                                                                                                                                                                                  ⁢                                                              Ls                                ,                                p                                                                                                              ⁢                                                      P                                                          0                              ⁢                              p                                                                                ⁢                                                      m                            p                                                    ⁢                                                                                    f                                                              p                                ⁢                                                                                                                                                                                        ⁡                                                          (                              t                              )                                                                                                                          ]                                        2                                                              ]                        ⁢                          ⅇ                                                -                  α                                ⁢                                                                  ⁢                L                                                                        (        5        )            
The summations in (5) are over the parameters of the “n” transmitters in the system. It is important to note that equations (1) through (5) are to be interpreted as being in the optical domain. Therefore the powers in these equations, including the distortions (CSOs, CSOp, CTBs, and CTBp) are optical powers and not electrical (or RF) power levels.
The Raman induced crosstalk and nonlinear distortions are more pronounced when the wavelengths are located near the zero dispersion wavelength of the optical transmission media through which the signals are co-propagating (i.e. the optical fiber). In the case of a near zero dispersion system the optical pump and signal waves are propagating at nearly identical group velocities through the media. The zero dispersion wavelength of a transmission media refers to the wavelength at which an optical signal will have no change in (inverse) group velocity with respect to changes in its optical frequency. The zero dispersion wavelength differs for different transmission media. In this case, the relative positions of the waves with respect to one another will remain nearly fixed throughout the length of the transmission media. Thus, if the signals Si and Sj are at or near the zero dispersion wavelength, they will largely maintain their relative phase with respect to one another. Hence, with very little walk off occurring between the optical channels the Raman induced crosstalk and distortions can build up along the fiber in a constructive manner. The dispersion will generally increase as the wavelength difference between the optical signal and the zero dispersion wavelength increases. If the signals Si and Sj are located at wavelengths far displaced from the zero dispersion wavelength, their relative phases will change as they propagate down the transmission path. The levels of Raman induced crosstalk and distortions are much lower in the nonzero dispersion scenario because, as the signals walk away from one another, it becomes more difficult for the crosstalk and distortions to build up constructively along the fiber length.
Accordingly, it is desirable to have a method and apparatus for reducing the levels of Raman induced crosstalk and distortions that arises among the individual channels comprising a WDM optical system. This is particularly true in the case of a system utilizing optical channels that are located near the zero dispersion wavelength of the transmission medium.